Especially the one that splits up into 3 mirror images and they start throwing you back and forth the room like a ragdoll.
Also, while doing the main quest-line, do you remember that dwarven ruin you are sent to, to get a certain famous item?
Did you actually EXPLORE that huge cave? OMG, the level designers who did the design of that place need to be given a medal.
Blackreach is freaking awesome.